A piano soundboard is a sheet of spruce wood shaped to fit on the inner rim of the piano under the cast-iron plate and piano strings. The soundboard is not flat. It has a convex or inverted bowl shape known as the crown. It is this crown which gives a piano its unique sound. In crafting a piano the soundboard and the rim which supports it are carefully shaped and sized to produce the precise crown which renders the characteristic sound of that piano. Nevertheless, due to slight differences in size and shape, variations in mounting techniques, and wood qualities, all held to the highest possible standards, there will be a variation in the nature of the sound. Some pianos will have the characteristic sound while others, wholly within acceptable limits, will not. After manufacture during the life of the piano, temperature, humidity, shock and age will affect the tension and crowning of the soundboard and change the sound.